The two isozymes of heart glutamate aspartate transaminase (mitochondrial and supernatant) differ in cytological localization and in chemical composition, nevertheless, they catalyze the same overall chemical transformation. A detailed study of the individual interaction of substrates and enzymatic inhibitors with the active site of each isolated isozyme will be carried out, taking advantage of the unique spectral characteristics of the free enzyme and enzyme-substrate complexes. Special attention will be given to: 1) The primary events on the enzyme's surface, 2) the sequence of events following the initial ligand interaction with each isozyme and 3) the subsequent transformations to release product. The similarities of the amino acids at their active site(s), as well as possible cooperative effects of these sites, will be compared. Organismic and ontogenic distribution of each isozyme will also be examined. We hope that this coordinated approach will help in the understanding of the structure and function relationship of the isozymes and lead to insight into their physiological significance. We will also utilize the knowledge gained by our studies to design new assays for the simple detection of these clinically significant isozymes.